An infatuation that develops out of close-quarters operation with another person.

"I swear I'm in love with this girl from Philosophy class. I've been sitting next to her for like three weeks and it's obvious we're meant for each other."

"Well do you know anything else about her? I bet you've just made up a life outside of class for her in your head. You're getting caught up in a short-sighted proximity infatuation man, just wait a couple weeks after class is over and you won't even remember her."

Constant close quarters lead to intense attraction.
This was first explained by Zelda Gilroy to Dobie Gillis in the 1959-1963 TV series, The Many Loves Of Dobie Gillis, except Zelda called it "Propinquity."